Structural Aspects of Dendritic Polymers
Hyperbranched polymers as a special class of dendritic polymers are occupying rapidly different application fields mainly as specialty polymers. In addition to high functionality and advantageous rheological features, they can be easily prepared using simple synthetic procedures starting from well-accessible multifunctional monomers. The statistical polymerization process however leads to multiple distributions, i.e. of molar mass, degree of branching and chemical structure which make their physicochemical characterization difficult. Furthermore direct application of the branching calculations of Stockmayer and Zimm to the dendritic polymer class is not possible due to their significantly higher branching density.
Well-defined model polymers help us to understand the structure-property relationship and to find new ways for a general description of the scaling properties of hyperbranched polymers. An example of our recent work are precisely synthesized polyesters, well defined in their molar mass, branching degree and chemical structure, which allow to theoretically describe the spatial dimensions and solution properties of statistically branched polymers. Polycondensation procedures based on the same AB2 monomers and accompanied by protection and deprotection steps lead to a variety of different branching degrees. Hence, monoprotected ABB* monomers give 0% degree of branching, e.g. linear polymers, whereas different ratios of AB2 to ABB* result in series of branching degrees up to the statistically achievable 50%.
Using bi-protected AB*2 monomers the terminal and linear functional units of the hyperbranched structures can be modified leading to pseudo-dendrimers of a core-shell type. Thereby two effects are achieved, an increase in the degree of branching up to 100% and an increase in the number of functional groups. Deprotection and repeated postmodification lead to higher generations.Investigations by static and dynamic light scattering, viscosity, hyphenated chromatographic separations, MD simulations and last but not least SANS help us to understand and describe the scaling characteristics of the dendritic architectures. While an increase in statistical branching leads smoothly to more compact structures in solution according to viscosity investigation, increasing generation number of pseudo-dendrimers rapidly decreases their solution viscosity. This behaviour resembles those known for dendrimers, where increasing segmental density is accompanied by a minor size change.
Essential further information on self-similarity and fractal dimensions of these rather small molecules (< 10nm) we obtain using SANS in combination with dynamic light scattering. On top of that, SANS provides us an insight into the local molecular structure by interpretation of the high momentum transfer values. In this way it is possible to show that the monomer contribution as well as the polydispersity have a significant influence on the particle scattering factor of hyperbranched polyesters. Further interpretation of the SANS results led to the first visualisation of the increasing segmental density with the branching degree.
Interestingly, the solution properties of these multifunctional polymers are only marginally influenced by the type of functional groups. This fact was repeatedly observed comparing hydrophobic and hydrophilic end groups. Further systematic variation of the flexibility of the monomeric units shows additional strong influence on the molecular properties of dendritic polymers in solution. Thus, flexible, hyperbranched aliphatic polyesters are getting compact with increasing molar mass, just like dendrimers. In contrast, aromatic-aliphatic polyesters possessing rather stiff monomer units show a linear increase in viscosity and size with the molar mass, as expected for a statistical coil. These investigations are supported by MD simulations which show the influence of solvent on the compactness of the dendritic structure, the latter is determined by the monomer type.
This research topic is realized within projects supported by the SMWK, DFG and as a part of a DPI project. Collaborations with ILL Grenoble and University Freiburg, contributed significantly to the progress in understanding the structure of dendritic polymers in solution. The SANS investigations on a series of differently branched polyesters as well as of pseudo-dendrimers are currently edited for publication. As a main author I am preparing the book Hyperbranched polymers: Macromolecules in between of deterministic linear chains and dendrimers structures for publication by the RSC next year.
- Sphere-like fourth generation pseudo-dendrimers with a hyperbranched core more Macromolecular Rapid Communications 33 (2012) 1440-1444
- Is the universal law valid for hyperbranched polymers? more Angewandte Chemie - International Edition 52 (2012) 4659-4663
- SANS investigation of global and segmental structures of hyperbranched aliphatic-aromatic polyesters more Macromolecules 45 (2012) 3177-3187
- Aromatic-aliphatic polyesters with tailored degree of branching based on AB/AB2 and ABB*/AB2 monomers more Macromolecules 43 (2010) 3268-3276
- Hyperbranched and highly branched polymer architectures - synthetic strategies and major characterization aspects more Chemical Reviews 109 (2009) 5924-5973